Bidirectional color printmodes with semistaggered swaths to minimize hue shift and other artifacts

ABSTRACT

Bidirectional scanning printheads discharge color-ink drops at ultrahigh resolution while scanning in each direction, to form color swaths on a print medium. The heads are at least partially aligned with respect to the longitudinal axis of the medium, so that the swaths at least partly overlap in that direction. An advance mechanism intermittently steps the print medium longitudinally, to enable displacement of successive swaths. A control system alternates (1) one full reciprocation of the heads, to discharge drops while scanning each way across the medium, with (2) each step of the advance mechanism. Preferably the heads print, while scanning each way, a respective generally fixed nonzero fraction of the total amount of each secondary color to be printed. In this way the overall appearance of each secondary is essentially consistent and partway between two appearances respectively produced by scanning two ways. This invention avoids the long-printzone drawbacks associated with full-height-staggered heads. Preferably the fractions for all the swaths are about equal, so that the consistent appearance for each secondary is essentially the average of two appearances respectively produced by scanning two ways. Various printmasks complete each swath in eight passes with four print-medium advances, or four passes and two advances, or two and one—in each case printing in every pass.

RELATED PATENT DOCUMENTS

Closely related documents include coowned U.S. Pat. Nos. 4,963,882entitled “PRINTING OF PIXEL LOCATIONS BY AN INK JET PRINTER USINGMULTIPLE NOZZLES FOR EACH PIXEL OR PIXEL ROW”, 4,965,593 entitled “PRINTQUALITY OF DOT PRINTERS”, 5,555,006 entitled “INKJET PRINTING:MASK-ROTATION-ONLY AT PAGE EXTREMES; MULTIPASS MODES FOR QUALITY ANDTHROUGHPUT ON PLASTIC MEDIA”, and 5,561,449, entitled “POSITION LEADING,DELAY, & TIMING UNCERTAINTY TO IMPROVE POSITION & QUALITY INBIDIRECTIONAL INKJET PRINTING”; as well as U. S. patent application Ser.No. 08/667,532, entitled “JITTER-FORM BACKGROUND CONTROL FOR MINIMIZINGSPURIOUS GRAY CAST IN SCANNED IMAGES” and now issued as U.S. Pat. No.5,859,928, U.S. Ser. No. 08/814,949 now issued as U.S. Pat. No.6,082,849, entitled “RANDOM PRINTMASKS IN A MULTILEVEL INKJET PRINTER”,Ser. No. 08/811,875 now issued as U.S. Pat. No. 6,142,605, entitled“BI-DIRECTIONAL COLOR PRINTING USING MULTIPASS PRINTMODES WITHSWATHALIGNED INKJET PRINTHEADS”, and as Ser. No. 08/811,788 now filedMar. 4, 1997 entitled “HIGH RESOLUTION INKJET PRINTING USING COLOR DROPPLACEMENT ON EVERY PIXEL ROW DURING A SINGLE PASS”. All these documentsin their entireties are hereby incorporated by reference into thisdocument.

FIELD OF THE INVENTION

This invention relates generally to machines and procedures for printingultrahigh-resolution color text or graphics on printing media such aspaper, transparency stock, or other glossy media; and more particularlyto a scanning inkjet machine and method that construct text or imagesfrom individual ink spots created on a printing medium, in atwo-dimensional pixel array. The invention employs print-mode techniquesto optimize ultrahigh-resolution color image quality vs. operating time.

BACKGROUND OF THE INVENTION

A previous generation of printing machines and procedures has focused onmixed resolution. These systems most typically have employed about 24pixels/mm (600 pixel dots per inch, or “dpi”) in a carriage scandirection transverse to the printing medium and 12 pixels/mm (300 dpi)in the print-medium advance direction longitudinal to the printingmedium—or 24 pixels/mm for black and 12 pixels/mm for chromatic colors,or relatively tall 12 mm (half-inch) pens for black ink and relativelyshort 8 mm (third-inch) pens for chromatic colors; or combinations ofthese and other operating-parameter mixtures.

These mixed-resolution systems have been of interest for obtainingeffectively very high quality printing with a minimum of developmentaldelay. In the continuing highly competitive development of inkjetprinter products, the mixed systems have served a very important rolebecause of many difficult problems associated with attaining a fullultrahigh resolution—for example 24 pixel/mm pens, 12 mm tall, for allcolorants in a color-printing system.

In the current generation of machines, interest has shifted to solvingthose many difficult problems. As will be seen, most of suchdifficulties have been recognized for many years, but tend to beaggravated in the ultra-high-resolution environment.

(a) Throughout and cost—In a sense many problems flow from these twoconsiderations, since essentially all the problems would evaporate if itdid not matter how slow or expensive a printer was. In practice,marketplace pressures have made it crucially important that a printer beboth competitively fast (even when printing in a “quality” mode) andcompetitively economical.

(b) Firing frequency—Thus for example high throughput in combinationwith high resolution pushes the capability of economical inkjet nozzlesto fire at a high enough repetition rate. An inkjet pen tends to be moststable in operation, and to work best for error hiding, at a low firingfrequency.

Horizontal resolution of 24 pixels/mm if printed all in a single pass,however, would require a rather high firing frequency—in fact, forcurrent-day technology, roughly twice the highest frequency of reliableoperation in an economical pen. This figure may be expected to changewith refinements in pens.

(c) Banding and pattern artifacts—These spurious image elements are wellknown in lower-performance printers, but like other problems can be evenmore troublesome in the newer generation of devices. It is known, forexample, that some banding effects can be reduced by printing highlystaggered (i. e., overlapping) swaths—but also that doing so reducesoverall throughput proportionately. (A different kind of visiblebanding, associated with hue shifts, will be discussed below.) Hence,again, high throughput tends to run counter to elimination of banding,and this conflict is aggravated by a requirement for printing atresolution that is twice as fine.

As to pattern defects, the design of dither arrays is a logical culpritand has previously received a great deal of attention in this regard,and may be considered highly refined. Yet heretofore some patterningpersists in high-resolution images printed under conditions which shouldyield the best possible image quality.

Theory suggests that no further advantage can be obtained through ditherredesign, and that solutions must be sought elsewhere. Discussion ofprintmasks in a following subsection of this document will take up thistheme again.

Generally speaking, tools for investigating this area heretofore havebeen inadequate.

(d) Color shift—One important approach to maximizing throughput is toprint bidirectionally. In a bidirectional-printing system the pens printwhile the carriage is traveling in each of its two directions—i. e.,across the printing medium, and back.

This technique is well known and successful for printing in monochrome.Workers skilled in this field have recognized, however, that forprinting in color a hue shift, or more precisely a color shift, arisesas between printing in the two directions.

The reason is that pens are traditionally arranged, physically, on theircarriage in a specific sequence. Therefore if two or more of the pensfire while the carriage is moving in one particular direction thedifferent ink colors are laid down one on top of another in acorresponding order—and while the carriage is moving in the oppositedirection, in the opposite order.

Usually the first inkdrop of two superposed drops tends to dominate theresulting perceived color, so that for example laying down magenta ontop of cyan produces a blue which is biased toward the cyan; whereasprinting cyan on top of magenta typically yields a blue which emphasizesmagenta. If successive separate swaths—or separately visible colorbands, subswaths—are printed while the pen is thus traveling in each oftwo directions, respectively, the successive swaths orsubswaths—therefore have noticeably different colors. Banding thatresults is often very conspicuous.

For this reason, previous artisans have striven to avoid printing of anysuperposition-formed secondary colors in more than one order, ever.Printers commercially available under the brand names Encad® andLasermaster®, in particular, employ a tactic that employs brute force toavoid sequence changes: the pens are offset, with respect to thevertical direction, or in other words longitudinally along the printingmedium.

They are offset by the full height of each nozzle array—posing, at theoutset, significant problems of banding (see discussion following) asbetween colors. Furthermore, in consequence of the full-height-offsetarrangement each of the trailing three pens must print over a colorsubswath formed in at least one previous scan—from one to three previousscans, depending upon which pen is under consideration.

This system advantageously maintains a fixed color sequence even inbidirectional printing. Use of full-height offset of the pens, however,makes a great sacrifice in other operating parameters. Morespecifically, the full-height staggered pens have a print zone that isfour color bands (subswaths) tall.

Necessarily the overall product size in the direction of printing-mediumadvance is correspondingly greater, as are weight and cost. In additionthe extended printzone is more awkward to manage in conjunction with around (i. e. cylindrical) platen.

Furthermore in this system it is considerably more awkward to hold theprinting medium consistently flat and without relative motion. Stillfurther, the trailing pen is overprinting a pixel grid that has alreadybeen inked by three preceding pens, and in a heavy-color region of animage this means that a considerable amount of liquid has already beenlaid down on the page, and the page has had a significant time to deformin response.

Substantial and uncontrollable intercolor registration problems may beexpected—particularly in view of the fact that this liquid-preloadingeffect is differential as between the several pens. In other words, itis present even for the second pen in the sequence, but suffered withprogressively greater severity by the third and fourth.

The Encad/Lasermaster systems use bidirectional printing for at leastthe so-called “fast” and possibly “normal” printing modes, but not forthe “best”-quality mode (which prints unidirectionally). Of course useof unidirectional printing as a best-quality printing mode incurs athroughput penalty of a factor as high as two. (Because the retrace maybe at a faster, slew speed the factor may be less than two.) Such apenalty can be very significant.

Thus the art has failed to deal effectively with hue shifts—animpediment to fully exploiting the potential of bidirectional printingas a means of enhancing through-put.

(e) Liquid loading—Hue shift, however, is not the only problem that isassociated with bidirectional printing. Another is microcoalescence.This may be regarded as a special case (particularly afflictingultrahigh-resolution operation) of excessive inking with itshistorically known problems—which are summarized below.

In still another difficulty, the tails or satellites of secondary-colordots, pointing in opposite directions, can generate textural artifactswhen the left-to-right order is reversed.

Excessive inking is a more-familiar problem. To achieve vivid colors ininkjet printing with aqueous inks, and to substantially fill the whitespace between addressable pixel locations, ample quantities of ink mustbe deposited. Doing so, however, requires subsequent removal of thewater base—by evaporation (and, for some printing media, absorption) andthis drying step can be unduly time consuming.

In addition, if a large amount of ink is put down all at substantiallythe same time, within each section of an image, related adversebulk-colorant effects arise: so-called “bleed” of one color into another(particularly noticeable at color boundaries that should be sharp),“blocking” or offset of colorant in one printed image onto the back ofan adjacent sheet with consequent sticking of the two sheets together(or of one sheet to pieces of the apparatus or to slipcovers used toprotect the imaged sheet), and “cockle” or puckering of the printingmedium. Various techniques are known for use together to moderate theseadverse drying-time effects and bulk- or gross-colorant effects.

(f) Prior print-mode techniques—One useful and well-known technique islaying down in each pass of the pen only a fraction of the total inkrequired in each section of the image—so that any areas left white ineach pass are filled in by one or more later passes. This tends tocontrol bleed, blocking and cockle by reducing the amount of liquid thatis all on the page at any given time, and also may facilitate shorteningof drying time.

The specific partial-inking pattern employed in each pass, and the wayin which these different patterns add up to a single fully inked image,is known as a “printmode”. Heretofore artisans in this field haveprogressively devised ways to further and further separate the inking ineach pass.

Larry W. Lin, in U.S. Pat No. 4,748,453—assigned to XeroxCorporation—taught use of a simple checkerboard pattern, which for itstime was revolutionary in dividing inking for a single image region intotwo distinct complementary batches. Lin's system, however, maintainscontact between pixels that are neighbors along diagonals and so failsto deal fully with the coalescence problem.

The above-mentioned U.S. Pat No. 4,965,593, which is in the name of MarkS. Hickman, teaches printing with inkdrops that are separated in everydirection—in each printing pass—by at least one blank pixel. The Hickmantechnique, however, accomplishes this by using a nozzle spacing andfiring frequency that are multiples of the pixel-grid spacing in thevertical and horizontal directions (i. e., the medium-advance and scanaxes respectively).

Accordingly Hickman's system is not capable of printing on interveninglines, or in intervening columns, between the spaced-apart inkdrops ofhis system. This limitation significantly hinders overall throughput,since the opportunity to print such further intervening information ineach pass is lost.

Moreover the Hickman system is less versatile. It forfeits the abilityto print in the intervening lines and columns even with respect toprintmodes in which overinking or coalescence problems are absent—suchas, for example, a high-quality single-pass mode for printing black andwhite text.

The above-mentioned U.S. Pat No. 5,555,006, which is in the name ofLance Cleveland, teaches forming a print-mask as plural diagonal linesthat are well separated from one another. Cleveland introducesprintmodes that employ plural such masks, so that (unlike Hickman) he isable to fill in between printed elements in a complementary way.

It is certainly not intended to call into question the Clevelandteaching, which represents a very substantial advance in the art—overboth Lin and Hickman. Cleveland's invention, however, in part is aimedat a different set of problems and therefore naturally has only limitedimpact on general overinking problems discussed here. In particularCleveland seeks to minimize the conspicuousness of heater-induceddeformation at the end of a page.

Thus even Cleveland's system maintains the drawback of inkdropcoalescence along diagonals and sometimes—since he calls for verysteeply angled diagonal lines which in some segments are formed byadjacent vertical pixels—even along columns.

Another ironic development along these lines is that the attempts tosolve liquid-loading problems through printmask tactics in some casescontribute to pattern artifacts. It will be noted that all theprintmodes discussed above—those of Lin, Hickman, Cleveland, and otherworkers not mentioned—are all highly systematic and thus repetitive.

For example, some printmodes such as square or rectangularcheckerboard-like patterns tend to create objectionable moire effectswhen frequencies or harmonics generated within the patterns are close tothe frequencies or harmonics of interacting subsystems. Such interferingfrequencies may arise in dithering subsystems sometimes used to helpcontrol the paper advance or the pen speed.

(g) Known technology of printmodes—One particularly simple way to divideup a desired amount of ink into more than one pen pass is thecheckerboard pattern already mentioned: every other pixel location isprinted on one pass, and then the blanks are filled in on the next pass.

To avoid horizontal “banding” problems (and sometimes minimize the moirepatterns) discussed above, a printmode may be constructed so that theprinting medium is advanced between each initial-swath scan of the penand the corresponding fill-swath scan or scans. This can be done in sucha way that each pen scan functions in part as an initial-swath scan (forone portion of the printing medium) and in part as a fill-swath scan.

This technique tends to distribute rather than accumulateprint-mechanism error which is impossible or expensive to reduce. Theresult is to minimize the conspicuousness of—or, in simpler terms, tohide—the error at minimal cost.

The pattern used in printing each nozzle section is known as the“printmode mask” or “printmask”, or sometimes just “mask”. The term“printmode” is more general, usually encompassing a description of amask—or several masks, used in a repeated sequence or so-called“rotation”—and the number of passes required to reach full density, andalso the number of drops per pixel defining what is meant by “fulldensity”.

Operating parameters can be selected in such a way that, in effect, maskrotation occurs even though the pen pattern is consistent over the wholepen array and is never changed between passes. Figuratively speakingthis can be regarded as “automatic” rotation or simply “autorotation”.

As mentioned above, some of these techniques do help to control theobjectionable patterning that arises from the periodic character ofprintmasks employed heretofore. Nevertheless, for the current newgeneration of ultrahigh-resolution color printers generally speaking thestandards of printing quality are higher, and a more-advanced control ofthis problem is called for.

(h) Conclusion—Thus persistent problems of firing frequency, hue shift,liquid loading, and pattern artifacts, counterbalanced against pervasiveconcerns of throughput and cost, have continued to impede achievement ofuniformly excellent inkjet printing. It may be added that certaincombinations of these difficulties are more readily controlled on oneand another printing medium; however, at least some of these problemsremain significant with respect to all industrially important printingmedia.

Thus, as can be seen, important aspects of the technology used in thefield of the invention remain amenable to useful refinement.

SUMMARY OF THE DISCLOSURE

The present invention introduces such refinement. In its preferredembodiments, the present invention has several aspects or facets thatcan be used independently, although they are preferably employedtogether to optimize their benefits.

In preferred embodiments of a first of its facets or aspects, theinvention is apparatus for printing a color image on a printing medium.The apparatus includes some means for scanning bidirectionally acrosssuch a printing medium, to discharge color-ink droplets at ultrahighresolution while scanning in each direction.

For purposes of generality and breadth in describing the presentinvention, these means will be called the “bidirectional scanningprinthead means” or sometimes more simply just the “scanning means” or“printhead means”. Through their scanning and printing action, thescanning means form swaths of such a color image on the printing medium.

These means include plural inkjet printheads to print plural colorsrespectively. These plural printheads are mutually at least partiallyaligned with respect to the longitudinal direction of the printingmedium. By virtue of this at least partial alignment, the pluralprintheads print respective color swaths which at least partiallyoverlap in that longitudinal direction.

The apparatus also includes some means for intermittent operation tomove such printing medium longitudinally. In this way these means enabledisplacement, when desired, of successive swaths along such medium.Again for breadth and generality these means will be called the“printing-medium advance means” or “medium advance means”, or simply“advance means”.

In addition the apparatus includes some means for alternating (1) onefull reciprocation of the scanning printhead means, to dischargecolor-ink droplets while scanning in each direction across such printingmedium and back, with (2) each operation of the printing-medium advancemeans. These means, again for like reasons, will be called the “controlmeans”.

The foregoing may constitute a description or definition of the firstfacet of the invention in its broadest or most general form. Even inthis general form, however, it can be seen that this aspect of theinvention significantly mitigates the difficulties left unresolved inthe art.

In particular, it may first be noted that the present invention avoidsthe full-height-offset-pen arrangement discussed in the “BACKGROUND”section of this document. Accordingly this invention is not subject tothe associated drawbacks of a long printzone—size, weight, cost,amenability to printing on a round platen, and print-medium flatness—orof differential liquid preloading, or susceptibility to intercolorbanding.

While avoiding all these problems, the alternation of a full printingreciprocation with a single advance—an arrangement which can be called“semistaggered” swaths (or subswaths)—can be used to entirely resolvethe hue-shift difficulties discussed earlier. It can also be used toeliminate or reduce certain pattern defects (sometimes seen as mottling)that arise from directional coalescence effects.

The invention accomplishes this by providing an equal number of passesin each direction—over each band or subswath of the print medium. Inthis way the invention tends to cancel out directional effects such asmentioned earlier, for example the visual artifacts that result frominkdrop satellites extending in different directions.

Although the invention as thus couched in its broadest formsignificantly advances the art of ultrahigh-resolution color inkdropprinting, nevertheless the invention is preferably practiced inconjunction with several additional features or characteristics thatmaximize the enjoyment of its benefits.

For instance preferably the control means include means for operatingthe printhead means to print, while scanning in each direction, arespective generally fixed nonzero fraction of the amount of eachsecondary color to be printed in a corresponding part of such image. Inthis case as will be understood the overall color appearance for eachsecondary color is substantially a consistent color appearance partwaybetween two color appearances respectively produced by scanning in twodirections.

It is further preferred that the respective generally fixed nonzerofractions for all the swaths be generally equal. In this case theconsistent color appearance for each secondary color is substantially anaverage of two color appearances respectively produced by scanning ineach of two directions.

Although it is preferable (from the standpoint of simplicity andconvenience) to print fixed fractions of the secondary colors asdescribed above, other ways of exploiting the semistaggered printmodecan be substituted. For instance the control means may cause theprinthead means to print, while scanning in a first direction, generallyall spots of two color secondaries to be printed in each swath.

The printhead means also can be caused to print, while scanning in asecond, opposite direction, generally all spots of a third colorsecondary, and of black, to be printed in each swath. In this way eachsecondary color is generally always deposited during scanning in arespective fixed, consistent direction, so that the overall colorappearance for each secondary color is substantially consistent.

Still another approach is that the control means cause the printheadmeans to print, while scanning in a first direction, generally allinkdrops of two color primaries to be printed in each swath. Theprinthead means also print, while scanning in a second, oppositedirection,. generally all inkdrops of a third color primary, and ofblack, to be printed in each swath.

As a result each primary color is generally always deposited duringscanning in a respective fixed, consistent direction, and the order ofprimary-color deposition is generally always consistent. Therefore theoverall color appearance for each secondary color, constructed bysuperposition of primary colors in consistent sequence, is substantiallyconsistent.

Specific preferred printmodes in accordance with these preferences aredescribed in a later section of this document.

A second aspect of the invention is an apparatus for printing a colorimage on a printing medium by applying ink of plural colors, includingin some regions superposed inkdrops of plural colors. It is to beunderstood that the superposed inkdrop colors are susceptible todifferences in resulting color appearance depending on order ofdeposition.

The apparatus includes bidirectional scanning print-head means generallyas introduced above in relation to the first facet of the invention—butdischarging (in at least some regions) superposed inkdrops of pluralcolors as just mentioned. Also as described above for the first aspectof the invention, the printhead means include at least partially alignedprintheads producing color bands that are longitudinally at leastpartially overlapping.

The apparatus also includes printing-medium advance means generally asdescribed for the first facet of the invention. In addition, theapparatus of this second aspect of the invention includes some means forminimizing color shifts due to order of deposition, while maximizingthroughput.

The foregoing may represent a definition or description of the secondaspect of the invention in its most general or broad form. Even in thisform it can be seen that this aspect of the invention too significantlymitigate a difficulty left unresolved in the art—in particular that ofcolor shifts associated with deposition order as described in the“BACKGROUND” section.

Nevertheless preferably the invention is practiced in conjunction withcertain further characteristics or features that additionally enhanceenjoyment of its benefits. For example preferably theminimizing-and-maximizing means comprise control means for alternatingone full reciprocation of the scanning printhead means, across suchprinting medium and back, with each operation of the printing-mediumadvance means.

In a third basic aspect or facet, the invention is a method for printinga color image on a printing medium. The printing is performed usingbidirectional scanning printhead means, printing-medium advance means,and control means.

The method includes the step of scanning the print-head meansbidirectionally across the printing medium, to discharge color-inkdroplets at ultrahigh resolution while scanning in each direction, andthereby form swaths of the color image on the printing medium. Thisscanning step includes printing plural bands of plural colorsrespectively, which plural bands are mutually at least partiallyoverlapping with respect to the longitudinal direction of the printingmedium.

A further step of the method is intermittent operation of theprinting-medium advance means to semistagger the swaths.

The foregoing may constitute a description or definition of theinvention in the third of its main aspects or facets. It has benefitsclosely related to those of the first two facets already discussed; andalso is amenable to preferred variants closely related to those of thefirst two aspects of the invention.

All of the foregoing operational principles and advantages of thepresent invention will be more fully appreciated upon consideration ofthe following detailed description, with reference to the appendeddrawings, of which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric or perspective exterior view of a large-formatprinter-plotter which is a preferred embodiment of the presentinvention;

FIG. 1A is a highly schematic block diagram of the same product,particularly showing key signals flowing from and to a digitalelectronic central microprocessor, to effectuate printing while the penstravel in each of two opposite directions;

FIG. 1B is a flow chart showing alternation of a full reciprocation ofthe pens with each advance of the printing medium, in some printmodes ofparticular interest;

FIG. 2 is a like view of a carriage and carriage-drive mechanism whichis mounted within the case or cover of the FIG. 1 device;

FIG. 3 is a like view of a printing-medium advance mechanism which isalso mounted within the case or cover of the FIG. 1 device, inassociation with the carriage as indicated in the broken line in FIG. 3;

FIG. 4 is a like but more-detailed view of the FIG. 2 carriage, showingthe printhead means or pens which it carries;

FIG. 5 is a bottom plan of the pens, showing their nozzle arrays;

FIG. 6 is a perspective or isometric view of an ink-refill cartridge foruse with the FIG. 4 and 5 pens;

FIG. 7 is a like view showing several refill cartridges (for differentink colors) according to FIG. 6 in, or being installed in, arefill-cartridge station in the left end of the case in the FIG. 1device;

FIG 8 is a very highly enlarged schematic representation of two usagemodes of the ultrahigh-resolution dot forming system of the presentinvention;

FIG. 9 is a schematic representation of generic printmask structures foruse in the present invention;

FIG. 10 is a flow chart showing operation of a printmask-generatingutility or development tool, implementing certain aspects of the presentinvention;

FIG. 11 is a diagram showing relationships between certain differenttypes of printmodes and printmasks;

FIG. 12 is a diagram schematically showing relationships between swathsprinted in a staggered or semistaggered printmode;

FIG. 13 is a diagram showing the elemental dimensions of a genericprintmask;

FIG. 14 is a diagram schematically showing relationships between threenotations or conventions for representing an exemplary set ofprintmasks;

FIG 15 is a set of diagrams showing pixels among which relationships areto be tested in the practice of location-rule aspects of the presentinvention;

FIG. 16 is a diagram showing pass numbers for printing of each pixel inan eight-by-eight pixel printmask which is called a “knight” pattern—foruse in eight-pass printmodes, with eight advances for glossy stock orfour for vinyl;

FIG. 17 is a like diagram for a sixteen-by-five (columns by rows)printmask, for use in a ten-pass printmode;

FIG. 18 is a like diagram for a different sixteen-by-five printmask, foruse in a five-pass printmode;

FIG. 19 is a like diagram for a sixteen-by-ten print-mask, for use in asix-pass, six-advance printmode;

FIG. 20 is a like diagram for a four-by-four print-mask, for use in afour-pass, four-advance printmode;

FIG. 21 is a like diagram for a four-by-four print-mask, for use in afour-pass, two-advance printmode;

FIG. 22 is a like diagram for a different four-by-four printmask, foruse in a two-pass, single-advance printmode;

FIG. 23 is a sample of a gray ramp printed using the presentinvention—in a so-called “fast” printing mode;

FIG. 24 is a like sample—but for a “normal” print-mode; and

FIG. 25 is a like sample—but for a “best quality” mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. BIDIRECTIONAL HIGH-RESOLUTION COLOR PRINTING WITH AT LEAST PARTIALLYALIGNED PENS

A preferred embodiment of the present invention is the first commercialhigh-resolution color printer/plotter to print bidirectionally withoutfull-height offset of the pens in the direction parallel to theprinting-medium advance. As will be seen, the invention gains severalimportant advantages by avoiding the extended printzone found in allbidirectionally operating high-resolution color printers heretofore.

More specifically, the present invention enables use of a mechanism thatis more compact, light and economical—and more amenable to operationwith a cylindrical platen of modest diameter. It is less subject tointercolor banding, differential distortion, and misregistration due todifferential liquid preloading under the several pens.

The printer/plotter includes a main case 1 (FIG. 1) with a window 2, anda left-hand pod 3 that encloses one end of the chassis. Within that podare carriage-support and -drive mechanics and one end of theprinting-medium advance mechanism, as well as a pen-refill station withsupplemental ink cartridges.

The printer/plotter also includes a printing-medium roll cover 4, and areceiving bin 5 for lengths or sheets of printing medium on which imageshave been formed, and which have been ejected from the machine. A bottombrace and storage shelf 6 spans the legs which support the two ends ofthe case 1.

Just above the print-medium cover 4 is an entry slot 7 for receipt ofcontinuous lengths of printing medium 4. Also included are a lever 8 forcontrol of the gripping of the print medium by the machine. Afront-panel display 11 and controls 12 are mounted in the skin of theright-hand pod 13. That pod encloses the right end of the carriagemechanics and of the medium advance mechanism, and also a printheadcleaning station. Near the bottom of the right-hand pod for readiestaccess is a standby switch 14. Within the case 1 and pods 3, 13 thecarriage assembly 20 (FIG. 2) is driven in reciprocation by a motor31—along dual support and guide rails 32, 34—through the intermediary ofa drive belt 35. The motor 31 is under the control of signals 31A from adigital electronic microprocessor 17 (FIG. 1A). In a block diagrammaticshowing, the carriage assembly is represented separately at 20 whentraveling to the left 16 while discharging ink 18, and at 20′ whentraveling to the right 17 while discharging ink 19.

A very finely graduated encoder strip 33 is extended taut along thescanning path of the carriage assembly 20, 20′, and read by an automaticoptoelectronic sensor 37 to provide position and speed information 37Bfor the microprocessor 15. (In the block diagram all illustrated signalsare flowing from left to right except the information 37B fed back fromthe sensor—as indicated by the associated leftward arrow.) The codestrip33 thus enables formation of color inkdrops at ultrahigh precision (asmentioned earlier, typically 24 pixels/mm) during scanning of thecarriage assembly 20 in each direction—i. e., either left to right(forward 20′) or right to left (back 20).

A currently preferred location for the encoder strip 33 is near the rearof the carriage tray (remote from the space into which a user's handsare inserted for servicing of the pen refill cartridges). Immediatelybehind the pens is another advantageous position for the strip 36 (FIG.3). For either position, the sensor 37 is disposed with its optical beampassing through orifices or transparent portions of a scale formed inthe strip.

A cylindrical platen 41—driven by a motor 42, worm 43 and worm gear 44under control of signals 42A from the processor 15—rotates under thecarriage-assembly 20 scan track to drive sheets or lengths of printingmedium 4A in a medium-advance direction perpendicular to the scanning.Print medium 4A is thereby drawn out of the print-medium roll cover 4,passed under the pens on the carriage assembly 20, 20′ to receiveinkdrops 18, 19 for formation of a desired image, and ejected into theprint-medium bin 5.

The carriage assembly 20, 20′ includes a previously mentioned rear tray21 (FIG. 4) carrying various electronics. It also includes bays 22 forpreferably four pens 23-26 holding ink of four different colorsrespectively—preferably yellow in the leftmost pen 23, then cyan 24,magenta 25 and black 26.

Each of these pens, particularly in a large-format printer/plotter asshown, preferably includes a respective ink-refill valve 27. The pens,unlike those in earlier mixed-resolution printer systems, all arerelatively long and all have nozzle spacing 29 (FIG. 5) equal toone-twelfth millimeter—along each of two parallel columns of nozzles.These two columns contain respectively the odd-numbered nozzles 1 to299, and even-numbered nozzles 2 to 300.

The two columns, thus having a total of one hundred fifty nozzles each,are offset vertically by half the nozzle spacing, so that the effectivepitch of each two-column nozzle array is approximately one-twenty-fourthmillimeter. The natural resolution of the nozzle array in each pen isthereby made approximately twenty-four nozzles (yielding twenty-fourpixels) per millimeter.

For resupply of ink to each pen the system includes a refill cartridge51 (FIG. 6), with a valve 52, umbilicus 53 and connector nipple 54. Thelatter mates with supply tubing within the printer/plotter refillstation (in the left-hand pod 3).

Each supply tube in turn can complete the connection to the previouslymentioned refill valve 27 on a corresponding one of the pens, when thecarriage is halted at the refill station. A user manually inserts (FIG.7) each refill cartridge 51 into the refill station as needed.

In the preferred embodiment of the invention, as illustrated in FIGS. 1Aand 1B, all print modes are bidirectional. In other words, consecutivepasses are printed 19, 18 while traveling in both directions,alternating left-to-right scans 17 with right-to-left 16.

Preferably black (or other monochrome) and color are treated identicallyas to speed and most other parameters. In the preferred embodiment thenumber of printhead nozzles used is always two hundred forty, out of thethree hundred nozzles (FIG. 5) in the pens.

This arrangement allows, inter alia, for software/firmware adjustment ofthe effective firing height of the pen over a range of ±30 nozzles, atapproximately 24 nozzles/mm, or ±30/24=±+1¼ mm, without any mechanicalmotion of the pen along the print-medium advance direction. Alignment ofthe pens can be checked automatically, and corrected through use of theextra nozzles. As will be understood, the invention is amenable to usewith a very great variety in the number of nozzles actually used. Thesystem of the preferred embodiment has three printing speed/qualitysettings, which determine resolution, number of passes to completeinking of each swath (or more precisely each subswath), and carriagevelocities as approximately:

best quality normal fast resolution (pixels/mm) 24 12 12 passes tocomplete swath  8 or 10 4 or 6  2 carriage velocity (cm/sec) 51 or 63 ½63 ½ 63 ½.

The varying choices indicated here are for correspondingly variousmedia—for example carriage velocity is 63½ cm/sec, except that 51 cm/secis used for best-quality printing on glossy stock. Resolution is thesame in both horizontal and vertical directions, i. e. row and columnspacings are the same so that pixels 57 (FIG. 8) are {fraction (1/24)}mm square for all settings.

All printing, even the lower-resolution (12 pixel/mm) operation, isactually controlled and produced on the high-resolution (24-by-24pixel/mm) grid. High-resolution printing, however, calculates the inkingfor each position in the grid independently, and implements that inkingindependently with one or more inkdrops 56 in each pixel.

Low-resolution printing instead calculates the inking only for everyother position in the grid (along each of the perpendicular axes ordimensions) and implements that inking with one or more double-height,double-width compound inkdrop structures 58—each made up of a two-by-twoassemblage of individual inkdrops. Since calculations are done for onlyhalf the rows and half the columns, the number of points calculated isjust one quarter of all the points in the grid.

2. RANDOMIZED MASKS

(a) General discussion—A printmask is a binary pattern that determinesexactly which inkdrops are printed in a given pass or, to put the samething in another way, which passes are used to print each pixel. In aprintmode of a certain number of passes, each pass should print—of allthe inkdrops to be printed—a fraction equal roughly to the reciprocal ofthat number.

As a practical matter, however, printmasks are designed to deal with thepixels to be addressed, rather than “printed”. The difference resides inthe details of an individual image which determine whether eachparticular pixel will be printed in one or another color, or left blank.

Thus a printmask is used to determine in which pass each pixel will beaddressed, and the image as processed through various other renditionsteps will determine whether each addressed pixel is actually printed,and if so with what color or colors. The printmask is used to, so tospeak, “mix up” the nozzles used, as between passes, in such a way as toreduce undesirable visible printing artifacts discussed earlier—banding,etc.

Whereas prior attention has focused upon dither masks as the sources ofpatterning and other artifacts, the present invention attempts toisolate the contributions of printmasks to these problems—and to theirsolutions. In particular this invention pursues the elaboration ofrandomization as a paradigm in printmasks.

This pursuit is totally contrary to all the wisdom of the artheretofore, which has been uniformly devoted to printmask modules anddesign techniques that are entirely systematic and repetitive—preciselythe opposite of random. Through this present contrarian approach asurprisingly high degree of success has been obtained.

(b) Masks according to the present invention—In the present preferredembodiment, a common printmask is used for each color (but that commonmask is different for different modes). Moreover the common mask usedfor each color is synchronized, in the sense that each pixel isaddressed in the same pass for all color planes.

As a very general rule, for preferred embodiments of the presentinvention two main kinds of masks may be recognized:

“one out of four” masks for most “normal” and “fast” print-qualitysettings—except average one out of six for some media, and

“one out of eight” masks, for the “best quality” setting—except anaverage one out of ten for matte.

The phrase “one out of four” means that each nozzle is fired atone-quarter of the maximum permissible frequency, and analogously for“one out of eight”.

Printmasks according to the present invention have been developed with afocus on single-field masks. A printmask “field” F (FIG. 9) is a maskunit, or building block, whose width measured in pixels is equal to thenumber of passes.

Thus a “single-field” printmask is one whose overall width W equals thenumber of passes. The width in pixels of a multiple-field mask can beintegrally divisible by the width as so defined (i e., by the number ofpasses), or can have an integral remainder R, called a residual.

(c) Software design tool used in implementing the present invention—Thebasic strategy for creating single-field print masks is massive randomiteration, using a simple algorithm implemented as a software designtool written in the “C” programming language and operating in anordinary general-purpose computer—with the results subject toapplication of location rules. The location, or dot-placement, rules aretaken up in a later subsection of this document.

The program begins with entry of a so-called “seed” 61 (FIG. 10) for useby the function “rando” of the “C” language. The program uses aninternal printmask data structure containing width, height, data,current line, current value, and temporal neighbors to current value.Within the first module 62, the algorithm generates the first line of aprintmask, one pixel value at a time, from the seed and the randofunction, and the location rules. Eventually each “pixel value” will beinterpreted as the pass number in which the corresponding pixel isaddressed.

Thus the “Generate_Line” function within the first module 62 as seenconsists of the “Generate_Value” function, using the rando functionseeded from the command line as already mentioned, combined with a test63 and a feedback path 64 in event of failure.

The line is tested against the location rules, either after completionof the entire line or after addition of each pixel value. Given that sofar there are no other lines of data, the number of restrictions in thefirst-line block 62 is minimal. If the line (or individual value,depending on the testing protocol) is not valid, it is discarded and anew one is generated. This procedure is iterated until a valid firstline has been created and can be printed out 65 for the designer'sreference.

Next the program enters the main loop 66. Operation here closelyparallels the first-line module 62, diverging in only three principalregards:

the testing 67 is more elaborate because of the greater number ofconstraints from already established lines,

testing at the bottom line of the mask is particularly elaborate sinceit includes a test against the already-established top line, which willbe vertically adjacent when the mask is stepped over the full pixelgrid, and

an extra test 68 is included to protect the system against cyclingindefinitely when earlier-established values or lines pose anintractable selection problem for later values or lines.

As to the last-mentioned test, it permits recycling 69—still within themain loop 66—up to a predefined limiting number of failures, but thendiscards the entire candidate mask and follows the loop path 71 to startthe whole procedure again. Such complete failures may seem catastrophicbut actually are very inexpensive in machine time and almostinsignificant in terms of designer time.

Ideally, the overall effect of the procedure described is to produceboth row randomization and column randomization. In other words, it isdesired that the pass used to print each row (considering, to take asimplified example, only pixels in a particular column) be selected atrandom; and that the row used to print each column also be selected atrandom.

As a practical matter the masks generated by this procedure may bedenominated “randomiz” or “semirandom”: they are developed through useof random numbers, but then subjected to exclusions which in many casesare quite rigorous. Naturally the finished array cannot be regarded astruly random, since a truly random array would have many coincidencesthat are forbidden in this environment.

During this preliminary generation stage the program is simplygenerating a very special numerical array, but naturally the array takeson solid physical meaning in the later usage stage—as the numericalpattern is applied directly to control electromechanical operation ofthe printer.

The algorithmic procedure described has been used to makeeight-by-fifteen-pixel, eight-pass masks as part of preferredembodiments of the present invention, and some smaller masks too as willbe seen. It is very generally characteristic of the most successfulmasks, used for the “best quality” settings in ultrahigh-resolutionbidirectional color printers/plotters, that they are much larger thanprintmasks employed heretofore. Some masks used in the preferredembodiment of the invention are sixteen pixels wide and one hundredninety-two pixels tall—that is, the width 87 (FIG. 13) is sixteen pixelsand the height 88 is one hundred ninety-two pixels.

Very small masks, and particularly very simple ones such as that in FIG.21, do continue to have a place in resolving fast-mode requirements forthe relatively less temperamental printing media. Such masks are easy towork out by hand since the number of possibilities is quite small;accordingly the algorithmic approach has generally not been used for thevery small masks.

(d) Designer participation to perfect the masking for eachoperating-parameter set—The objective of these mask-generation exercisesis to elaborate randomized masking as a means for minimizing patterningartifacts and excess inking. The proof of this pudding thus cannot beobtained from the degree of randomization actually imparted to givenmasks, for the artifacts and overinking problems involved are-complexproducts of interactions between ink and media.

These interactions at the present writing are, with some exceptions,inordinately unpredictable. The physics of microcoalescence, thechemistry of inks and paper sizing, the biochemistry of some fiber-basedprint media and the electrostatics of others that are synthetic, allintertwine to produce a morass of variability in observablebehaviors—which often seems to go beyond the merely bewildering to thetruly temperamental. Accordingly the present invention relies heavilyupon human observation, and human esthetic evaluation, to selectactually useful solutions from those generated. The selection is basedon actual trial of the printmasks, as applied in printing of bothsaturated and unsaturated images.

Massive trial and error is involved in finding the best: some masks arebetter for some combinations of medium and quality/speed requirements,other masks for other combinations. Through extensive testing theinvention has settled upon three masks, for use at differentprint-quality settings, for each medium.

(e) Further refinement—As noted earlier, a randomized printmaskaccording to the present invention may, as a finished product, be ratherfar from random. The relatively stringent location rules (see section 4below) which are responsible for this particularity in selection are inpart due to firing-frequency constraints or the strength of coalescencein modern inks.

In the foreseeable future with advances in the relevant electronic andchemical systems a relaxation of both these types of constraint may beexpected. The result should be a greater degree of randomness in theprintmask generating process—and more-random patterns in the actualfinished-product masks.

Such developments will lead to continuingly improved print quality. Suchquality improvements may in particular materialize in, for example, evenimages printed using the fast-mode settings.

Another area of contemplated extension of the present work is in thedirection of multiple-field masks with no “residual” as previouslydefined; then multiple-field masks with a residual; and alsocustomizable dot-placement rules. All such refinements are within thescope of the invention as defined by certain of the appended claims.

3. SEMISTAGGERED PRINTMODE

(a) Terminology—For present purposes a “'swath” is a print regiondefined by the number of available and actually used nozzles of a penand the actually used width of a printing medium. In a “single pass”print mode 76 (FIG. 11), all nozzles of a pen are fired to providecomplete coverage for a given swath of image data.

Single-pass modes have the advantage of speed, but are not optimal interms of coalescence or ink loading. Therefore swaths are often printedin multipass modes 77, with each swath containing only part of theinking needed to complete an image in some region of the print medium.

In this case only a fraction of all the nozzles fire in each pass, ineach column of the pixel grid. Multipass color printing heretofore hascreated swaths that were either superimposed 78 or staggered 80.

In the case of superimposed swaths 78, a sequence of printmasks is used,one after another, all to print one common portion of the image. Onlythen is the page advanced—by the full swath height, since inking hasbeen completed for the subject portion—and then the nextsuperimposed-swath portion of the image is printed.

Given that all the swaths are printed one on top of another, each passmust be different or “asymmetric” to achieve complete coverage withoutduplication. This scheme tends to result in banding and is not highlyvalued for the current generation of printer products.

In the case of staggered swaths 80 a constant pixel offset is used tosuccessively advance the pen during printing, through some fraction ofthe swath height. By virtue of this repetitive stepping of the printingmedium, resulting printed swaths overlap in the direction ofprint-medium advance. Either symmetric masking 82 or asymmetric masking83 may be adapted to staggered swaths 80—as explained at some length inthe Cleveland Pat. No. 5,555,006 mentioned earlier.

An example appears very schematically in FIG. 12. Here the verticaladvance 85—by successive small off-sets 86—represents successiveplacements of swaths 1-4, by virtue of the printing-medium advance (inthe opposite direction to the arrow 85).

(In this drawing the slight horizontal offsets between swath rectangles1, 2, . . . are included only to make it easier to visualize thesuccessive swath positions. In actual printing of course there is nosuch horizontal displacement.)

As in the case of superimposed swaths, each staggered swath containsonly part of the inking needed to complete an image strip—but now, sincethe swaths are not all laid down in the same place, that “strip” is onlya fraction of the area of any one of the swaths. Ignoring end effects attop and bottom of a page (or sheet, or length) of the medium, thatelemental “strip” in which the number of passes needed for completioncan be evaluated may be called a “subswath” or “band”.

Thus for instance in FIG. 12 the only subswath that is actually shown ascomplete—i. e., with inking from the four swaths needed to complete itsimage elements—is the strip actually containing the numeral “4”,adjacent to the offset marking “86”. The top three subswaths (containingthe numerals “1” through “3”, as drawn) require earlier-formed swathsfor completion; while the bottom three (containing no numerals) requirelater-formed swaths for completion.

The height 86 of a subswath or band is ordinarily equal to the offsetdistance of any two successive offset swaths—i. e., the verticaldistance by which they are staggered. This offset, which again isnormally a fraction of the overall swath height, is often expressed inpixels.

(b) A hybrid mode, novel to color printing—The present invention employsa bidirectional color printmode 79, incorporating a hybrid of thesuperimposed swaths 78 and staggered swaths 80 which may be called“semistaggered”. In this system the pens print while traveling in eachdirection, and the printing medium is advanced as for staggeredswaths—but not after every pass, rather instead only after every otherpass.

More specifically, the medium advances 42A (FIG. 1B) after each fullreciprocation 19, 18 of the pen carriage, and the distance of thatadvance is most commonly a fraction of the height of each used nozzlearray (i. e., swath). As to successive passes between which the mediumis not advanced, the operation is as for superimposed swaths; as tosuccessive passes between which the medium is advanced, the operation isas for staggered swaths.

As explained earlier, semistaggering of swaths is readily exploited tosubstantially eliminate hue shifts and also eliminates or greatlyminimizes certain directional types of coalescence artifacts. It isamenable to use with printmasks that minimize overinking problems.

An example of multipass staggered-swath masking employed in preferredembodiments of the present invention may be represented in any of atleast three equivalent notations 91, 92, 93 (FIG. 14). The mostgraphically plain notation 92 is essentially a representation of a partof the pixel grid, as addressed in each of four passes.

In that notation each pass is represented by a separate rectanglecontaining numerals (ones and zeroes) in rows and columns. Each row ineach rectangle is part of a row in a particular portion of the overallpixel grid of the image, and each column in each rectangle is part of acolumn in the same portion of the overall pixel grid. In operation theserectangles are repeatedly stepped, so that the pattern is reused manytimes; however, in most preferred high-quality printmodes the mask ismuch larger than the example, so that considerably less repetition ispresent.

All four rectangles represent the same pixel-grid portions. Thus eachnumeral (“1” or “0”) inside the rectangles represents what happens at aspecific pixel in part of the overall pixel grid.

In these representations a “1” means that that particular pixel isaddressed—i. e., printed if there is anything to print—during the passrepresented by the rectangle under consideration.

Hence in the first pass the system addresses the pixel second from theleft in the top row, the pixel at the far right in the second row, andthat at the far left in the third row. It also addresses the pixel thirdfrom the left in the bottom row.

Exactly the same thing is shown by the numbers 91 at left of thediagram—i. e., the numerals “4”, “1”, “8” and “2”—which are simplyhexadecimal (or decimal) encodings of the patterns within the rectanglesread as binary numbers. In other words, “0100” binary is equal to “4” inhexadecimal or decimal notation, “0001” is equal to “1” in hex, “1000”to “8” in hex and “0010” to “2”.

Again the same is shown by picking out the numerals “1” inside thesingle rectangle 93 at the right. Each such “1” means that the pixelposition where the “1” appears is printed in pass number one—the topmostof the rectangles 92 already discussed.

Correspondingly the numerals “3142” across the top row of the rectangle93 mean that the pixel positions in which these numerals appear areaddressed in, respectively, passes number three, one, four and two. Thissystem can be related to the central rectangles 92 by noting which ofthose rectangles 92 has a “1” in the same respective pixel positions:the third rectangle for the top-left pixel, first rectangle for thesecond pixel, etc.

Although as noted above the advance distance is ordinarily a fraction ofthe swath height, a two-pass/one-advance mode such as shown in FIG. 21requires a full-height advance. In such a case successive swath pairsare abutted, leading to some banding; however, FIG. 21 does represent anoptimal fast mode for certain media.

4. LOCATION RULES

As mentioned earlier, one ideal objective is row and columnrandomization, to minimize patterning while maintaining throughput. Onthe other hand, another important ideal objective is wide separationbetween inkdrops laid down in the same pass—and also in temporallynearby passes—to minimize puddling while maintaining through-put.

These objectives are inconsistent, since truly random pass assignmentswould occasionally produce nearer neighbors than consistent with goodliquid management. What is desired is an optimum tradeoff between thetwo ideals. Most-highly preferred embodiments of the present inventionfollow these rules:

no immediate neighbors in any direction—horizontal, vertical ordiagonal;

no more than one pixel in any row, within the entire width of theprintmask;

no more than one pixel in any column, within the entire height of theprintmask;

no immediate neighbors in any direction in the immediately precedingpass; and

adherence to the no-immediate-neighbors rule across the seams of twovertically abutted masks, or horizontally abutted masks, or both.

The first of these rules derives from well-known coalescence or puddlingconsiderations, i. e. from concerns about overinking. It focuses uponimmediately adjacent horizontal neighbor 4 (FIG. 15)—where the centerpixel 95 in the diagram represents a pixel currently underconsideration—and also immediately adjacent vertical neighbor 5, andimmediately adjacent diagonal neighbor 3.

The second rule actually arises from firing-frequency limitations, asmentioned earlier, but also of course helps to minimize overinking byspreading printed dots as much as possible. It focuses on“firing-frequency neighbors” 2.

For current pens the maximum firing frequency is 7.5 kHz, and a designobjective is to stay at least a factor of two below that value. In mostof the selected masks the effective frequency is four to eight timeslower than that value, for a very fully effective margin of error.

The third rule is directed to overinking, and focuses on “verticalfrequency” neighbors 1. The fourth rule is concerned with the same, butin regard to possibly-incompletely-dried inkdrops deposited in theimmediately preceding pass—i. e., what may be called a“horizontal-temporal” neighbor 6, “vertical-temporal” neighbor 8, and“diagonal-temporal” neighbor 7.

The fifth and final rule is essentially the same as the first butfocused upon the regions where adjoining masks come together.

Thus positions 1 and 2 are influenced primarily by pen parameters(firing capabilities), while the other positions are critical for inkand media artifacts.

Generally it has been possible to satisfy all the criteria stated, ineight-pass modes (i. e., printmasks with at least eight rows).Inadequate flexibility is available in six- and four-pass modes; hencesome relaxation of the rules is required. For example in a fourpass modethe firing is one in four rather than one in eight.

5. ACTUALLY SELECTED MASKS

FIGS. 16 through 21 display the masks chosen from those randomlygenerated, after testing as described above. As mentioned earlier, someof the smaller masks were generated manually but still with attention toselection of the numbers at random.

The mask of FIG. 16 was found to produce best printed image quality forglossy stock, and also for a vinyl printing medium, and accordingly wasselected for use at the “best” mode setting for those two media. It isfamiliarly called a “knight” printmask because the pixels assigned toeach pass appear, relative to one another, two pixels over and onedown—like the move of the piece called a “knight” in the game of chess.

The FIG. 17 mask when tested produced best image quality on matte stock,and FIG. 18 best image quality when backlit—in other words, used foroverhead projection or simply in a backlit display frame as in sometypes of advertising displays. It is a “two hundred percent of ink”mode, in which all normal inking is doubled. The mask of FIG. 17 is usedat the “best” print-quality setting on matte, and FIG. 18 for backlittransparencies.

The FIG. 19 mask is used in the “normal” setting for glossy, heavy matteand vinyl. Inspection of the information shows clearly that several ofthe location rules are relaxed.

FIG. 20 shows a mask used for “normal” printing on backlit transparencymedia (at two hundred percent inking), and also for “fast” printing onglossy and vinyl stock—all at four passes and four advances. FIG. 21 isused for “normal” printing on matte, with four passes and two advances;and FIG. 22 is used at the “fast” setting on a matte medium, with twopasses and one advance.

The mask of FIG. 22 at a glance may seem trivial but actually is theproduct of considerable thought. As shown in FIG. 5, each printhead ismade with—pursuant to convention—two rows of nozzles, the two rows beingoffset by half the nozzle spacing in each row. If a printmode happens tocall for addressing, say, all odd-numbered nozzles in one pass and alleven in the next pass, this seemingly arbitrary specification has aphysical significance which may be unintended: in heavily inked regions,what will fire is in the first pass the entire left-hand column ofnozzles and then, in the second, the entire right-hand column.

As a practical matter of constructional detail, pens are generally madewith one common ink-supply channel supplying all the ink chambers in theleft-hand row, and another distinct common channel supplying all thechambers in the right-hand row. Firing all odd or all even nozzlestherefore selectively drains only one or the other supply channel,tending through liquid-flow impedance effects to aggravate any tendencyof some nozzles to fire weakly. These may be, for example, the nozzlesfurthest from the channel source inlets—or those which happen to havebeen made with aperture sizes low-within-tolerance.

The mask of FIG. 22 calls for firing in a single pass (pass “1”, forexample) two vertically adjacent pixels in the upper right corner of themask—which means two nozzles in immediate succession in the numberingsequence. These are, physically, one adjacent nozzle in each of the twocolumns. Thereby liquid loading is distributed equally between the twosupply channels, not concentrated in one or the other. The same sharingof the hydraulic loading is seen whichever pass is considered.

This thinking was enough to include the FIG. 22 mask among those whichshould be subjected to comparative testing. In that testing it was foundthat the FIG. 22 mask provided slightly better image quality than itsnatural alternative, a plain checkerboard pattern. Accordingly the FIG.22 mask has been adopted for use—but only on matte stock, for whichcoalescence problems are at a minimum.

6. OPERATION USING THE SELECTED MASKS

In operation the masks are simply called up automatically. They areselected by the combination of print-quality and print-medium settingswhich a user of the printer/plotter enters at the control panel 12, asverified by the display 11.

Each pass number in a particular cell of a mask is applied directly bythe system central processor, to cause the carriage drive 31,medium-advance drive 42-44, encoder sensor 37, and pen nozzles (FIG. 5)with associated firing devices all to cooperate in implementing thepass-number indication. That is, they cooperate in such a way that allthe pixels corresponding to that particular cell will be printed duringthe indicated pass—if there is anything to print in those pixelsrespectively.

The physical results may be seen directly in FIGS. 23 through 25, whichshould indicate clearly the relative quality levels available—withcomplementary speeds of printing—through use of the present invention.

In those of the accompanying claims which are directed to apparatus, averbal convention has been adopted to make particularly distinct andclear which features mentioned in the claims are elements of the claimedinvention and which features (first mentioned in the preamble) areinstead recited as parts of the environment in which the inventionexists and operates. Specifically, in referring back to parts of theenvironment, consistently the word “such” is used instead of the word“the” or “said”. A common, unitary antecedent is intended: in otherwords, the term “such” is to be understood (as are “the” and “said”) asa definite article, referring back to a particular element of theenvironment that is the same element whenever referred to.

The above disclosure is intended as merely exemplary, and not to limitthe scope of the invention—which is to be determined by reference to theappended claims.

What is claimed is:
 1. Apparatus for printing a color image on aprinting medium; said apparatus comprising: bidirectional scanningprinthead means for scanning bidirectionally across such printingmedium, to discharge color-ink droplets at ultrahigh resolution whilescanning in each direction, and thereby form swaths of such color imageon such printing medium in rows and columns, said rows being spacedapart by a pixel-row spacing; said printhead means including pluralinkjet print-heads to print plural colors respectively, which pluralprintheads are mutually at least partially aligned with respect to thelongitudinal direction of the printing medium; whereby the pluralprintheads print respective color swaths which at least partiallyoverlap in that longitudinal direction, and wherein each printhead hasnozzles spaced apart at the same spacing as the pixel rows;printing-medium advance means for intermittent operation to move suchprinting medium longitudinally to enable displacement, when desired, ofsuccessive swaths along such medium; and control means for alternating(1) one full reciprocation of the scanning printhead means, to dischargecolor-ink droplets while scanning in each direction across such printingmedium and back, with (2) each operation of the printing-medium advancemeans.
 2. The apparatus of claim 1, wherein: the control means comprisemeans for operating the printhead means to print, while scanning in eachdirection, a respective generally fixed nonzero fraction of the amountof each secondary color to be printed in a corresponding part of suchimage; and the overall color appearance for each secondary color issubstantially a consistent color appearance partway between two colorappearances respectively produced by scanning in two directions.
 3. Theapparatus of claim 2, wherein: the respective generally fixed nonzerofractions for all the swaths are generally equal; and said consistentcolor appearance for each secondary color is substantially an average oftwo color appearances respectively produced by scanning in twodirections.
 4. The apparatus of claim 1, wherein; the control meanscomprise means for operating the printhead means to print: whilescanning in a first direction, generally all spots of two colorsecondaries to be printed in each swath, and while scanning in a second,opposite direction, generally all spots of a third color secondary, andof black, to be printed in each swath; and each secondary color isgenerally always deposited during scanning in a respective fixed,consistent direction; and the overall color appearance for eachsecondary color is substantially consistent.
 5. The apparatus of claim1, wherein: the control means comprise means for operating the printheadmeans to print: while scanning in a first direction, generally allinkdrops of two color primaries to be printed in each swath, and whilescanning in a second, opposite direction, generally all inkdrops of athird color primary, and of black, to be printed in each swath; and eachprimary color is generally always deposited during scanning in arespective fixed, consistent direction; the order of primary-colordeposition is generally always consistent; and the overall colorappearance for each secondary color, constructed by superposition ofprimary colors, is substantially consistent.
 6. The apparatus of claim1, wherein: the control means comprise means for operating the printheadmeans to complete each swath in eight passes, with printing in eachpass, and with four printing-medium advances.
 7. The apparatus of claim6, wherein: the operating means provide best quality of printing onvinyl.
 8. The apparatus of claim 7, wherein: the operating means imposean eight-by-eight “knight”-pattern printmask.
 9. The apparatus of claim1, wherein: the control means comprise means for operating the printheadmeans to complete each swath in four passes, with printing in each pass,and with two printing-medium advances.
 10. The apparatus of claim 9,wherein: the operating means provide normal quality of printing on mattestock.
 11. The apparatus of claim 10, wherein: the operating meansimpose a four-by-four printmask with the pattern: 2  1  4  3 4  3  2  11  2  3  4 3  4  1 
 2.


12. The apparatus of claim 1, wherein: the control means comprise meansfor operating the printhead means to complete each swath in two passes,with printing in each pass, and with one printing-medium advance. 13.The apparatus of claim 12, wherein: the operating means provide fastprinting on matte stock.
 14. The apparatus of claim 13, wherein: theoperating means impose a four-by-four printmask with the pattern: 1  2 1  2 1  2  1  2 2  1  2  1 2  1  2 
 1.


15. Apparatus for printing a color image on a printing medium byapplying ink of plural colors, including in some regions superposedinkdrops of plural colors, said superposed inkdrop colors beingsusceptible to differences in resulting color appearance depending onorder of deposition; said apparatus comprising: bidirectional scanningprinthead means for scanning bidirectionally across such printing mediumto discharge color-ink droplets at ultrahigh resolution in pixel rowsand columns while scanning in each direction, including in some regionssuperposed inkdrops of plural colors, said superposed inkdrop colorsbeing susceptible to differences in resulting color appearance dependingon order of deposition and said rows being spaced apart by a pixel-rowspacing; said printhead means including plural inkjet print-heads toprint plural colors respectively, which plural printheads are mutuallyat least partially aligned with respect to the longitudinal direction ofthe printing medium; whereby the plural printheads print respectivecolor bands which at least partially overlap in that longitudinaldirection, and wherein each printhead has nozzles spaced apart at thesame spacing as the pixel rows; printing-medium advance means forintermittent operation to move such printing medium longitudinally toenable displacement, when desired, of successive swaths along suchmedium; and means for minimizing color shifts due to order ofdeposition, while maximizing throughput.
 16. The apparatus of claim 15,wherein: the minimizing-and-maximizing means comprise control means foralternating one full reciprocation of the scanning printhead means,across such printing medium and back, with each operation of theprinting-medium advance means.
 17. The apparatus of claim 16, wherein:the control means comprise means for operating the printhead means toprint, while scanning in each direction, a respective generally fixednonzero fraction of the amount of each secondary color to be printed ina corresponding part of such image; and the overall color appearance foreach secondary color is substantially a consistent color appearancepartway between two color appearances respectively produced by scanningin two directions.
 18. The apparatus of claim 16, wherein: the controlmeans impose a constraint, as stated below, in terms of a triad ofcontrol colors, said triad being either: three color secondaries, orthree color primaries; the control means comprise means for operatingthe printhead means to print: while scanning in a first direction,generally all spots of two colors of said triad to be printed in eachswath, and while scanning in a second, opposite direction, generally allspots of a third color of said triad, and of black, to be printed ineach swath; and each color of the triad is generally always depositedduring scanning in a respective fixed, consistent direction; and theoverall color appearance for each of the triad color is substantiallyconsistent.
 19. A method for printing a color image on a printingmedium, using bidirectional scanning printhead means comprising pluralprintheads each with nozzles spaced apart at a nozzle spacing,printing-medium advance means, and control means; said method comprisingthe steps of: scanning the printhead means bidirectionally across theprinting medium, to discharge color-ink droplets at ultrahigh resolutionwhile scanning in each direction, and thereby form swaths of the colorimage on the printing medium in a pixel grid of rows and columns, saidrows being spaced apart by a pixel-row spacing equal to said nozzlespacing; wherein said scanning step includes printing plural bands ofplural colors respectively, which plural bands are mutually at leastpartially overlapping with respect to the longitudinal direction of theprinting medium; and intermittent operation of the printing-mediumadvance means to semistagger the swaths.
 20. The method of claim 19,wherein: the scanning step comprises operating the printhead means toprint, while scanning in each direction, a respective generally fixednonzero fraction of each secondary color to be printed in acorresponding part of such image; and the overall color appearance foreach pair of generally adjacent spots of each secondary color issubstantially a consistent color appearance partway between two colorappearances respectively produced by scanning in two directions.